Rubber tires, such as pneumatic tires, include many components, such as, for example, sidewalls. Sidewalls are continuously subjected to distortion under normal road operating conditions. The sidewalls are subjected to extensive continuous flexing and can crack under such conditions. In addition to flex cracking, sidewalls are also subjected to atmospheric chemical action such as ozone attack. The overall effect is that the sidewalls may erode and degrade. The sidewall may even separate from the tire carcass during use, leading to tire failure.
To reduce the problems caused by ozone attack and flex cracking, tire manufacturers add chemical protectants to the general purpose rubbers used in tire sidewalls. The problem with these protectants is that they tend to be fugitive and can cause staining when in contact with white sidewalls. In some cases, particularly in white sidewalls, polymer blends have been used to effect improvements in ozone and flex resistance.
Published International application PCT/US91/05666 filed Aug. 9, 1991 discloses a tire sidewall composition comprising a single layer prepared from a blend of a halogenated copolymer of isoolefin and para-alkylstyrene with one or more general purpose rubbers.
U.S. Pat. No. 5,376,438 to Costemalle et al discloses multi-layer tire sidewalls having an outer layer prepared from a blend of halogenated isomonoolefin/para-alkylstyrene copolymer and general purpose rubbers. The inner layer comprises general purpose rubber. The sidewalls are said to exhibit good ozone resistance and fatigue crack resistance as well as a reduction in staining and discoloration. This patent is hereby incorporated herein by reference in its entirety.
In the prior art, the halogen content of the halogenated isoolefin/para-alkylstyrene copolymers generally varied proportionately with the para-alkylstyrene content, e.g. higher para-methylstyrene content was used to get a higher halogen content. If the copolymer was excessively halogenated, some undesirable dihalo benzylic substitution would occur. Thus, excess para-methylstyrene was usually used. In tire sidewall compositions of the prior art, a relatively high bromine level was needed to achieve adhesion and ozone resistance, but such high bromination levels led to relatively tight cures and crack propagation. It would be desirable to be able to use a rubber blend of the halogenated isoolefin/para-alkylstyrene copolymer that has both good adhesion and ozone resistance, as well as flex crack resistance and cure properties.